The present invention relates to an image forming method using heat or thermal transfer and more particularly to an image forming method using a thermal transfer, wherein a dye-receptive layer or an image protective layer is transferred uniformly so that an excellent image can be formed.
Heretofore, various thermal transfer methods have become known. Among these, a method uses a heat or thermal transfer film comprising a sublimable dye layer carried as a recording substance on a base film or substrate such as a paper or a plastic film. According to this method, full-color images are transferred from the thermal transfer film onto a dye-receptive layer of a print sheet. In the practice of this method, a thermal head of a printer is used as a heating means. Thus a large number of color dots of three or four colors are transferred onto the dye-receptive layer of the print sheet by heating for an extremely short time. By these multicolor dots, a full-color image of an original is reproduced on the print sheet.
According to the above described method, the material of the print sheet has been limited to materials such as a plastic sheet possessing dyeability or a paper provided beforehand with a dye-receptive layer. That is, images could not be formed directly on readily available materials such as ordinary paper. This has been a problem. It is possible, of course, to form an image on ordinary paper if a dye-receptive layer is previously formed thereon. In general, however, such a procedure entails high cost and is difficult to apply to materials that are generally ready made such as postcards, memo sheets, letter-papers, and papers for report writing.
Various methods have been proposed to solve these problems. One such method aims to form in a simple manner a dye-receptive layer on only necessary portions for forming images on a print sheet of a ready-made material such as ordinary paper by using a receptive layer transfer film, as disclosed in U.S. Pat. No. 5,006,502. Further, as disclosed in U.S. Pat. No. 4,522,881, in order to improve the durability of a dyed image formed in the above described manner, the depositing of a protective layer transfer film comprising a transparent resin on the dyed image surface has been proposed.
A method intended to simplify the operational procedure comprises the following steps. On the surface of a long substrate or base film, dye layers respectively of yellow, cyan, magenta, and, if necessary, black are formed in sequence. On the same surface of the substrate film, a transferable dye-receptive layer and/or a transferable protective layer are/is provided. First, the dye-receptive layer is transferred onto a print sheet. Then, dyes of the respective colors are transferred onto the dye-receptive layer thereby to form a full-color image. When required, a protective layer is transferred onto the image surface.
In each of the above described methods, a dye-receptive layer is transferred by means of a thermal head. In the case where the print sheet is a sheet lacking surface smoothness such as an ordinary paper sheet or a postcard, the adhesion of the dye-receptive layer with the print sheet becomes a problem, which gives rise to a further problem in that a uniform receptive layer is not transferred.
This problem can be solved by increasing the thermal energy imparted to the print sheet and the dye-receptive layer. However, if transferring of the dye-receptive layer under this condition of high thermal energy is continued, heat will accumulate within the printer and, as a consequence, will give rise to various problems. One such problem is the matting (roughening) of the surface of the dye-receptive layer. Another problem is the fusing of the heat-resistant layer or its substrate sheet to the thermal head, whereby the smooth feeding of the thermal transfer film is obstructed or the transfer film is torn. Still another problem is apt to occur in the case where a releasing layer or parting layer has been formed between the dye-receptive layer and the substrate film of the transfer film. In this case, the parting layer may melt to cause defective separation of the dye-receptive layer, which will then be prevented from being completely transferred. In consideration of the accumulation of heat within the printer as mentioned above, the imparting of the thermal energy at a low rate from the beginning may appear to be a solution. However, if this measure is taken, defective transfer will occur during the initial period of the transfer, whereby there will be much untransferred parts or incompletely transferred portions. Another problem is the possibility of transfer of the dye-receptive layer with uneven edges.
The above described problems occur also in the case where a protective layer is transferred from a protective layer thermal transfer sheet onto the image surface formed on the dye-receptive layer.